Electrically assisted power steering system for motor vehicles

ABSTRACT

An electrically supported power steering system for motor vehicles includes an input shaft, which is mechanically linked to a steering wheel for transmitting a steering torque required for steering of wheels to be steered. An output element is mechanically linked to the wheels to be steered. An electric motor, through which a servo force can be applied directly or indirectly to the output element, is arranged on the power steering system. The input shaft and the output element are connected to one another via a torsion bar. A detection unit for detection of a torque acting on the input shaft and of other steering-specific parameters includes a single pulse generator designed as a magnet ring and at least one magnetoresistive sensor.

FIELD OF THE INVENTION

The present invention relates to an electrically supported powersteering system for motor vehicles. The power steering system includesan input shaft mechanically linked to a steering wheel and used totransmit a torque required for the steering of wheels to be steered. Anoutput element is mechanically linked to the wheels. A servo motorthrough which servo power can be applied directly or indirectly to theoutput element is arranged on the power steering system. The input shaftand the output element are connected to one another via arotation-elastic element so that a limited torsion movement is possiblebetween the input shaft and the output element. A contactless detectionunit is used for contactless detection of the direction and intensity ofa steering torque acting on the input shaft and of othersteering-specific parameters. The detection unit includes at least onesensor respectively connected to the input shaft or the output element.

BACKGROUND INFORMATION

Such a power steering system is described, for example, in GermanPublished Patent Application No. 38 44 578. In this power steeringsystem, two drums made of non-magnetic material are respectivelyconnected to an input shaft or an output element. The drums are providedwith magnetizable media on their periphery or on an end face, so thatthey form alternating magnetic north and south poles. By measuring thedifference in the rotation angles of the two drums, the torque isdetermined using magnetoresistive elements.

The servo support of an electric steering system can be controlled bydetecting the torque. If the servo support of an electric steeringsystem is controlled only as a function of the torque, this is perceivedas unusual in a driving operation, because the friction conditions inthat case are different from those of a hydraulically supported powersteering system. There is no “feedback” of the steering rate from thesteering torque. In order to compensate for this disadvantage, thesteering rate must be detected. In order to detect the steering rate, anadditional sensor is provided in conventional electric steering systems,for example, in German Published Patent 37 11 854. This sensor has a DCtacho-generator, for example, which generates a direct current having avoltage adjusted to the steering rate. Such a steering rate sensorinvolves a relatively high cost for detecting the steering rate.

SUMMARY

It is an object of the present invention to improve the control of anelectrically supported power steering system. In particular, it shouldallow steering-specific parameters to be detected inexpensively and in asimple manner.

The above and other beneficial objects of the present invention areachieved providing a power steering system in which a detection unit ofthe generic power steering system contains a single pulse generatordesigned as a magnet ring, which is provided with alternating magneticnorth and south poles on its peripheral surface or on its end face, sothat both the relative position of the input shaft and the outputelement with respect to one another, as well as the change in theposition of the input shaft, are detected via allocated sensors. Thisdesign results in low assembly costs. In addition, the manufacturingcosts of the steering system are reduced due to the small number ofcomponents.

Thus, the electrical connection between rotating parts of the sensorsand a stationary sensor housing can be established via a flat spiralspring. This allows the signals to be transmitted in a simple andreliable manner.

Instead of a flat spiral spring, a wiper unit may also be used for theelectrical connection between rotating parts of the sensors and astationary sensor housing.

In order to also be able to detect the absolute steering angle in asimple manner, a gear, through which a plurality of revolutions of theinput shaft or of the output element can be converted into smallrotation or translation movements, is arranged on the output element.The gear contains a thread connected to the output element or the inputshaft and a threaded nut, the threaded nut being guided by an axialguide in an axially movable but non-rotatable manner. In addition, thethreaded nut is connected to a magnet that generates a signal with itsaxial movement for the absolute steering angle.

Using such a gear, a magnetoresistive sensor, which is used fordetecting steering-specific parameters such as steering angle, steeringrate, and steering acceleration, and another magnetoresistive sensor,which is used for detecting the absolute steering angle, are arranged ona common circuit board and work together with a common electronic unit.

The sensor units for detecting the steering-specific parameters torque,steering direction, steering angle, absolute steering angle, steeringrate, and steering acceleration may be combined in a single unit. Thisin turn results in low assembly costs.

When using an electric motor as a servo motor, the measured parametersmay be used for controlling the motor speed in a simple manner,resulting in better control quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first embodiment of a powersteering system according to the present invention.

FIG. 2 is a cross-sectional view of a second embodiment of a powersteering system according to the present invention.

FIG. 3 is a cross-sectional view of a third embodiment of a powersteering system according to the present invention.

FIG. 4 is a cross-sectional view of an electric motor of the powersteering system taken along the line A—A shown in FIG. 3.

DETAILED DESCRIPTION

The present invention is described with reference to the example of apower steering system having a rack-and-pinion gear. The invention may,however, also be used with other power steering systems to the sameeffect such as, for example, systems having ball-and-nut gears orsystems where the servo unit is arranged in the steering column.

A pinion 3 connected to a pinion shaft 2 is rotatably mounted in asteering housing, identified hereinafter as housing 1. Pinion 3represents an output element, which is mechanically linked to wheels tobe steered (not illustrated) via a rack 4. In the case of a ball-and-nutsteering system, a threaded spindle replaces pinion 3. Pinion shaft 2 isconnected to an input shaft 6 of the steering gear via a torsion bar 5.As an alternative to torsion bar 5, another rotation-elastic element maybe used.

Rack 4 is drivably connected to a servo motor designed as an electricmotor 7 via a reduction gear (not illustrated). The servo motor mayinclude a hydraulic motor.

In one part of housing 1, which is the upper part next to input shaft 6in the example embodiment illustrated in FIG. 1, an enlargement 8 isformed to accommodate a detection unit 10.

Detection unit 10 contains a sensor housing 11 that includes a two-partdesign in this embodiment. A magnetoresistive sensor 12, referred tohereinafter as an MR sensor, having an electronic unit 13 on a circuitboard 14, is mounted in sensor housing 11. MR sensor 12, electronic unit13, and circuit board 14, together form a sensor unit 15 that rotatessecurely with input shaft 6.

A pulse generator in the form of a magnet ring 16 is arranged on apinion shaft 2. Magnet ring 16 is provided on its peripheral surface oron its end face with alternating magnetic north and south poles. As analternative to magnet ring 16, the pulse generator may be designed as abar magnet. Magnet ring 16 works together with MR sensor 12.

A flat spiral spring 17 represents an electrical connection betweenstationary sensor housing 11 and rotating sensor unit 15. Flat spiralspring 17 is a flexible, coiled flat strip wire.

A torque applied to input shaft 6 produces a torsional angle betweeninput shaft 6 and pinion shaft 2. This torsional angle is a measure ofthe torque and is measured by sensor unit 15. MR sensor 12 only measuresin the area of a pole.

A second sensor unit 18, which is arranged on one end face of sensorhousing 11, belongs to detection unit 10. Thus, sensor unit 18 isdesigned to be stationary. Sensor unit 18 contains an MR sensor 20 andan electronic unit 21, both mounted on a circuit board 22. Second sensorunit 18 is arranged so that its MR sensor 20 works together with thesame magnet ring 16 as first MR sensor 12. Sensor unit 18 measures themovement of magnet ring 16 via a plurality of poles. The steering angle,the steering rate, and the steering acceleration may be derived from themeasuring signal.

The two sensor units 15 and 18 deliver a measuring signal from which therespective steering direction may be derived. The two sensor units 15and 18, together with flat spiral spring 17 and sensor housing 11, formdetection unit 10 as a unitary component. Measuring signals may beforwarded from detection unit 10 to an electronic control unit (notillustrated) via a common cable 23.

In the example embodiment illustrated in FIG. 2, pinion shaft 2 havingpinion 3 is drivably connected to electric motor 7 via a toothed gear orworm gear 24. A worm wheel 25 of worm gear 24 is mounted on pinion shaft2. A worm 26, which is connected to an output shaft 27 of electric motor7, works together with worm wheel 25.

In this example embodiment, a wiper unit 28 is used for the electricalconnection between stationary sensor housing 11 and rotating servo unit15 instead of flat spiral spring 17.

Neither the use of wiper unit 28 nor that of flat spiral spring 17 isbound to the position of electric motor 7. Therefore, both types ofelectrical connection may be used as alternatives in the embodimentsillustrated in FIGS. 1 and 2.

The remaining components of the embodiment according to FIG. 2substantially correspond to the parts of FIG. 1.

A third embodiment embodiment is explained with reference to FIGS. 3 and4. A multiple magnet ring 30 is arranged on worm 26, i.e., output shaft27 of electric motor 7. This magnet ring 30 works together with anotherMR sensor 31. MR sensor 31, together with an electronic unit 32 and acircuit board 33, forms a sensor unit 34. Magnet ring 30 must bepositioned on worm 26 and MR sensor 31 must be positioned on circuitboard 33 so that they are located at a precisely defined distance 35from one another. Magnet ring 30 may be positioned on the opposite sideof the gear engagement with worm 26, i.e., away from electric motor 7.

The main components of electric motor 7 are a rotor 36 and a stator 37.Stator 37 is secured to the housing; Rotor 36 is connected to outputshaft 27 and therefore to worm 26. The rotor speed is measured viamagnet ring 30. A mechanical connection exists between magnet ring 30and pinion 3 via worm 26 and worm wheel 25 of worm gear 24. Thus, sensorunit 34 can measure the quantities steering angle, steering rate, andsteering acceleration, in addition to the rotor speed. In this case, theresolution increases, because the transmission ratio of worm gear 24multiplies the signals.

Input shaft 6 and pinion 2 are rotated more than 360° in normal steeringoperation, i.e., about two to four revolutions. Sensor units 18 and 34cannot detect the absolute value of this angular range. Therefore a gear38 is arranged on the output element of the power steering system,namely on pinion shaft 2, through which a plurality of revolutions ofinput shaft 6 and pinion shaft 2 may be converted into small rotationalor translational movements. One embodiment of such a gear 38 isillustrated in FIG. 3. As illustrated in FIG. 3, a thread 40 isconnected to pinion shaft 2 through which a corresponding threaded nut41 performs a small translation movement when pinion shaft 2 is rotated.In the embodiment illustrated in FIG. 3, thread 40 is arranged on magnetring 16, while threaded nut 41 carries a magnet 42. Thus, magnet 42performs the same axial movement as threaded nut 41. This axial movementis detected by another MR sensor 43 and is a measure of the absolutesteering angle. MR sensor 43, together with MR sensor 31, is mounted oncircuit board 33. The two MR sensors 31 and 43 share electronic unit 32.

In order to prevent threaded nut 41 from rotating together with thread40, threaded nut 41 is guided by an axial guide 44 in an axially movablebut non-rotatable manner.

An an alternative to the example embodiments illustrated in FIGS. 1through 3, sensor unit 15 may be connected to pinion shaft 2 instead ofinput shaft 6. Magnet ring 16 and magnet 42 would then be arranged oninput shaft 6. In this arrangement, the steering angle and/or thesteering rate may be measured directly on input shaft 6. The torsionalangle between input shaft 6 and pinion 3 does not affect the measurementresult.

Magnet ring 16 or the corresponding bar magnet, magnet ring 30 magnet 42may be replaced with other pulse generators such as, for example, atoothed gear or a rack. This has no effect on the function of the sensorsystem.

The important thing is that a single magnet ring is sufficient theoperation of the power steering system. The parameters torque, steeringrate, and steering acceleration may be detected with the embodimentsillustrated in FIGS. 1 and 2. For greater steering comfort, for example,for improved return motion, the steering function may be extended withthe detection of the absolute steering angle, for example, using gear 38and additional magnet 42.

What is claimed is:
 1. An electrically supported power steering systemfor a motor vehicle, comprising: an input shaft mechanically linked to asteering wheel of the motor vehicle, the input shaft being configured totransmit a steering torque to steer the wheels of the motor vehicle; anoutput element mechanically linked to the wheels of the motor vehicle tobe steered; a torsionally flexible element connecting the input shaftand the output element, the torsionally flexible element providing alimited torsion movement between the input shaft and the output element;a servomotor configured to apply a servo power to the output element;and a detection unit configured to contactlessly detect a direction andan intensity of the steering torque imparted on the input shaft and atleast one steering-specific parameter, the detection unit including atleast one sensor connected to one of the input shaft and the outputelement, the at least one steering-specific parameter including at leastone of a torque, a steering rate and a steering acceleration; whereinthe detection unit includes a single pulse generator, the single pulsegenerator including a magnet ring, at least one of a peripheral surfaceand an end face of the magnet ring including alternating magnetic northand magnetic south poles so that a relative position between the inputshaft and the output element and a change in position of the one of theinput shaft and the output shaft are detectable by the at least onesensor.
 2. The electrically supported power steering system according toclaim 1, wherein the servomotor is configured to apply the servo powerdirectly to the output element.
 3. The electrically supported powersteering system according to claim 1, wherein the servomotor isconfigured to apply the servo power indirectly to the output element. 4.The electrically supported power steering system according to claim 1,further comprising a stationary sensor housing and a flat spiral spring,the at least one sensor including a rotating part, the flat spiralspring electrically connecting the rotating part and the stationarysensor housing.
 5. The electrically supported power steering systemaccording to claim 1, further comprising a stationary sensor housing anda wiper unit, the at least one sensor including a rotating part, thewiper unit electrically connecting the rotating part and the stationarysensor housing.
 6. The electrically supported power steering systemaccording to claim 1, further comprising a gear arranged on the one ofthe output element and the input shaft, the gear configured to convert aplurality of revolutions of the input shaft into one of a smallrotational movement and a small translational movement.
 7. Theelectrically supported power steering system according to claim 6,further comprising a threaded nut and an axial guide, the axial guidebeing configured to guide the threaded nut axially and non-rotatably,the gear including a thread connected to the output element and thethreaded nut.
 8. The electrically supported power steering systemaccording to claim 7, further comprising a magnet connected to thethreaded nut, the magnet configured to generate a signal in accordancewith an axial movement and in accordance with an absolute steeringangle.
 9. The electrically supported power steering system according toclaim 7, further comprising: a first magnetoresistive sensor configuredto detect the at least one steering-specific parameter; a secondmagnetoresistive sensor configured to detect an absolute steering angle;and a common electronic unit including a common circuit board, the firstmagnetoresistive sensor and the second magnetoresistive sensor beingarranged on the common circuit board.
 10. The electrically supportedpower steering system according to claim 1, wherein the servo motorincludes an electric motor.
 11. The electrically supported powersteering system according to claim 10, wherein the at least one sensorof the detection unit is configured as a single unit, the at least onesensor being configured to detect the steering torque, a steeringdirection, a steering angle, an absolute steering angle, the steeringrate, the steering acceleration and a rotor speed of the electric motor.